This invention pertains to an overspeed safety device, particularly to an overspeed safety device for a pneumatic rotation motor, comprising a valve element guidingly supported in a bore in the motor housing and arranged to block at least partly the air supply to the motor when shifted from a normally open position to a closed position. Such device also comprises an activating means coupled to a rotating part of the motor and arranged to be shifted by inertia forces from a normal rest position to an active position at a predetermined motor speed level, and a trip means supporting the valve element in the open position and arranged to be triggered by the activating means.
The main object of the invention is to create an overspeed safety device for a pneumatic motor that is simple and compact in structure yet reliable in operation.
One example of previously known overspeed safety devices of the above type is described in U.S. Pat. No. 2,977,931. In this prior art device a cylindrical valve element is rotatably supported in a bore in the motor housing to control an air inlet passage, and an activating means is arranged to engage and move the valve element to closed position at a predetermined speed level.
A drawback inherent in this known device is the large contact surfaces between the valve element and the housing necessary to guide a rotative element. These surfaces tend to increase the frictional resistance between the valve element and the housing, especially after a long inactive period when corrosive and/or other influence upon the surfaces has taken place.
Another example is shown in G.B. Patent No. 1,366,482 where a valve spindle is arranged to be moved axially by a spring to block an axial air inlet passage when released by a trip means at activation of a fly-weight on the motor rotor.
This known device suffer from the drawback of being able to control just a small inlet area, because an increased area would require a stronger spring to accomplish a closing movement of the valve element against the inlet pressure. A heavier spring load would in turn have a negative influence upon the action of the trip means, or would even make the use of a trip means impossible.
The present invention intends to create an improved overspeed safety device by which the above problems are avoided.
A preferred embodiment of the invention is described below in detail with reference to the accompanying drawings.